FR2851112A1 - Audiovisual sequence distributing method, involves transmitting nominal flow having digital information apt to allow reconstruction of nominal flow, and calculating synthesis of flow according to two flows generated from server - Google Patents

Abstract

The method involves modifying a part or all of succession of packets that have been substituted by packets of same structure but whose contents have been modified. A nominal flow having digital information apt to allow the reconstruction of the nominal flow is separately transmitted. A synthesis of flow is calculated at the nominal format according to two flows generated from a server towards a recipient equipment. Independent claims are also included for the following: (a) An equipment for the production of a video bit stream for the implementation of audiovisual sequence distribution method (b) an equipment for the operation of a video bit stream (c) a system for the transmission of a video bit stream.

Description

SECURE DEVICE FOR BROADCASTING, RECORDING AND

  ON-DEMAND VISUALIZATION OF AUDIO-VISUAL WORKS IN MPEG-2 TS TYPE

  The present invention relates to the processing, broadcasting, recording and secure viewing of audiovisual data and television programs, or more generally any multimedia program or sequence using a nominal stream format of MPEG type, by authorized users and provides a secure system for processing, broadcasting, delivery, recording, private copying and viewing of interactive programs or video or multimedia sequences.

  The general problem is to provide a device capable of securely transmitting a set of 15 films or programs of high visual quality in a format of MPEG-2 TS type ("Transport Stream" in English or transport stream), directly to a television screen and / or to be recorded on the hard disk of a box connecting the remote transmission network to the television screen, while preserving the audiovisual quality but avoiding any fraudulent use such as the possibility of making copies pirates of films or audiovisual programs recorded on the hard drive of the set-top box.

  With current solutions, it is possible to transmit films and audiovisual programs in digital form via radio, cable, satellite, etc. distribution networks. or via telecommunications networks such as DSL (Digital Subscriber Line) or BLR (Local Radio Loop) or via DAB networks (Digital Audio 30 Broadcasting). Furthermore, to avoid pirating of the works thus distributed, the latter are often encrypted by various means well known to those skilled in the art.

  In the prior art with respect to the present invention, the solution closest to our invention is described by document WO 00/60846 (Diva Systems Corporation). This invention provides a video on demand distribution solution 5 via a system of distributed servers. The protection of digital video content is based on an encryption system based on digital keys. The main innovation of this patent is to allow encryption and decryption of MPEG-TS streams, which is simplified in that an analysis of the MPEG stream is carried out beforehand in order to optimize the encryption according to the structure of the stream. . The computation time is indeed greatly reduced thanks to a selective and optimal encryption of the MPEG-TS stream. This analysis of the flow mainly consists in marking the TS packets containing information essential for a correct decoding of the video, in the MPEG sense, so that in a second encryption phase, only the useful data of the marked TS packets are encrypted . Thus, essential information 20 is not available for unauthorized decoding, and the stream cannot therefore be displayed, while retaining a reasonable calculation time for encryption. However, this solution does not make it possible to solve the security problem because the protection of the video stream relies on an encryption system based on digital keys, but for which the video stream transmitted to a receiving device contains, in an encrypted form, all the information originally present in the original video stream. Thus, the video stream received by the client equipment already contains all the information initially present in the unencrypted video stream, even if it is stored in a form which does not allow its use. A malicious user can process the encrypted video stream so as to put it back in a form usable by a conventional video decoder and can thus bypass the protection system.

  In order to prevent access to a compressed digital copy of a video program, another solution encountered in the prior art consists in adding protection to the video while it is still in analog form, before digital compression. Thus, to remove the associated protection, the video stream must be processed after the MPEG decoding, since it is in this form that the protection has been introduced, which implies that only a decompressed version of the unprotected stream is accessible. However, an initial uncompressed version of a video stream is difficult to use. With this in mind, the solution proposed by document EP 1 011 269 Ai (Mindport) is known, of the same type as that proposed by document WO 00/60846 in that it is also based on encryption based on digital keys. . In addition, this solution is not sufficient because the reconstructed video is not perfectly identical to the original video.

  Another solution is to prevent the possibility of viewing or making an uncompressed digital copy of the stream by conditioning the use of the video stream on obtaining protection information from a distributor equipment. With this in mind, the solution proposed by document EP 0 975 165 30 A2 (Sony Corporation) is known, similar to that of document WO 00/60846 in that it uses encryption based on digital keys. However, this solution is not sufficient to remedy the technical problem of security, because the information sent from the distributor equipment to the recipient equipment for the use of the video stream is information related to the limited reproduction rights of the video stream. . Thus, the video stream received by the client equipment again contains all the information initially present in the unprotected video stream.

  The invention relates in its most general sense to a method for the distribution of audiovisual sequences according to a nominal stream format complying with the MPEG-2 TS standard, composed of a succession of TS packets, characterized in that one proceeds , before the transmission to the client equipment, to an analysis of the flow to generate a first flow conforming to the MPEG-2 TS format, modified in that all or part of the TS packets constituting it have been replaced by packets of the same structure but the content of which has been modified, and a second stream of any format, comprising the digital information able to allow the reconstruction of said nominal stream, then to transmit separately the two streams thus generated from the server to the recipient equipment, and in that a summary of a stream in nominal format is calculated on the recipient equipment as a function of said first stream and of said second stream.

  Preferably, said substituted TS packets contain information relating to one or more video streams included in the TS stream.

  Advantageously, said analysis of the flow includes a step of determining the TS packets to be substituted as a function of their video content.

  According to a variant, said second stream contains the substituted TS packets originating from the nominal stream.

  According to another variant, the second stream respects the MPEG-2 TS format.

  According to a particular mode of implementation, said first stream has the same size in bytes as the original stream in MPEG-2 TS format.

  The invention also relates to equipment 10 for manufacturing a video stream for implementing the method, comprising at least one multimedia server containing the original video sequences and characterized in that it comprises a device for analyzing the stream video from said server to generate said two streams.

  The invention also relates to equipment for operating a video stream for implementing the method, comprising a standard MPEG-2 TS stream player, at least one recording interface [hard disk] intended for storing the content of said first stream and at least one display interface, characterized in that it comprises a means for re-composition of the original stream from said two streams.

  Finally, the invention relates to a system for the transmission of a video stream comprising an equipment for producing a video stream, at least one equipment for operating a video stream and at least one communication network between the production equipment and operating equipment (s).

  The present invention will be better understood on reading the description of a nonlimiting exemplary embodiment which follows, referring to the appended drawings o: * FIG. 1 describes the overall architecture of a system for the implementation of the method according to the invention * FIG. 2 represents a particular embodiment of the MPEG-type stream analysis and synthesis system according to the invention The invention relates to an audiovisual stream in MPEG-2 TS format containing at least a basic packaged video stream. The format of the packaged video stream 10 used must have the following characteristics: * this format must decompose the data into frames [frame], each frame comprising at least one complete digital image I (Intra); * each image I is broken down into macroblocks of a given size, said macroblocks themselves breaking down into blocks of a given size; * each image I contains correlation coefficients between the different blocks and / or macroblocks of the images I.

  In the following description, the example

  relates to an MPEG-2 TS stream containing an MPEG-2 video stream without this limiting the scope of protection. The invention however applies in the same way for MPEG-4 multimedia streams, directly in TS format or multiplexed in an MPEG-2 TS stream.

  The general principle of a process for securing an audiovisual stream is set out below.

  The objective is to authorize video-on-demand and pay-per-view services across all broadcast networks and local recording in the user's digital set-top box. The solution consists in permanently keeping a part of the recorded audiovisual program outside the user's home, in fact in the broadcasting and transmission network, this part being essential for viewing said audiovisual program on a screen. television or monitor type, but being of a very low volume compared to the total volume of the digital audiovisual program recorded by the user. The missing part will be transmitted via the broadcasting network (transmission) at the time of viewing of said pre-recorded digital audiovisual program at the user's.

  Most of the audiovisual stream, called "first stream" will therefore be transmitted via a conventional broadcasting network, while the missing portion called <second stream "will be sent on demand via a narrowband telecommunications network like conventional telephone networks. or cellular networks of the GSM, GPRS or UMTS type or by using a small part of a DSL or BLR type network, or even by using a subset of the shared bandwidth on a cable network.

  In the accompanying drawing, Figure 1 is a block diagram of a distribution system according to the present invention.

  FIG. 2 represents a particular embodiment of the system for analysis, protection and reconstruction of flows according to the invention.

  In FIG. 1, the video interface arrangement (8) is adapted to connect at least one display device, for example a monitor, a video projector or a television screen type device (6), to at least a broadband transmission and broadcasting network interface (4) and at least one telecommunications network interface (10). According to the present invention, this arrangement is composed of a module (8) mainly comprising, on the one hand, a processing unit suitable for processing, in particular reconstructing the original stream and decoding any MPEG type video stream according to a software program pre-loaded decoding and reconstitution, so as to display it, in real or delayed time, to store it, to record it and / or to send it over a telecommunications network and, on the other hand, at least one screen interface (7) and an interface for connection to a local or wide area network (5) and / or (9). The broadband transmission and broadcasting network (4) and the telecommunications network (10) can be merged into a single network.

  The hard disk of the module (8) can be used as a buffer memory for temporarily storing at least part of the program or of the video sequence to be displayed, in the event of delayed viewing or of limitation in the bandwidth of the transmission network. Viewing can be delayed or delayed at the request of the user or the portal (12).

  As shown in FIG. 1, the connection interface (5) is connected to a broadband transmission and broadcasting network (4) such as a modem, a satellite modem, a cable modem, a communication interface fiber optic line or radio or infrared interface for wireless communication.

  It is through this classic video broadcasting link that the content of audiovisual programs such as films will be transmitted. However, so as not to allow pirated copies to be made, before transmitting the audiovisual content from the server (1) or the portal 30 (12) it is planned to keep a small part of the audiovisual content in the portal (12).

  When viewing an audiovisual program in real time, this small part of the audiovisual content stored in the portal (12) will also be sent to the module (8) via the telecommunications network (10).

  Since the successive images of a video sequence contain a large number of identical visual elements, MPEG only records the elements which differ from the successive images. Thus, by modifying only an entire reference image I and retaining the digital information able to allow its reconstruction in the portal (12), the successive images which depend on this reference image I are also affected by the degradation, and it it is not necessary to make any modifications since they will cause the displayed flow to diverge due to the disturbances made to the 15 reference I images. The object of the MPEG-2 system part is to allow the combination of one or more elementary streams of video and audio data, as well as other types, to form isolated or multiple streams suitable for recording or recording. transmission. Subpart TS 20 of the system part of the MPEG-2 standard describes a data stream format for broadcasting audiovisual programs on networks exposed to transmission errors such as cable or satellite.

  The streams conforming to the TS (“Transport Stream” in English or transport stream) part of MPEG-2, the transport streams, are hereinafter called MPEG-2 TS streams.

  The operation which consists in forming an MPEG-2 TS stream from one or more elementary streams is called multiplexing, the reverse operation of extracting the elementary streams within an MPEG-2 TS stream is called demultiplexing. Each MPEG-2 TS stream consists of a succession of 188 byte packets. The size of these packets (otherwise called TS packets) is fixed. Each packet contains a header allowing in particular to identify its payload (or "payload" in English) as well as other digital data. The payload of the TS packets is used to transport the data to be broadcast, such as for example the elementary audio and video streams. The elementary streams multiplexed within an MPEG-2 TS stream are previously packaged so as to form packaged elementary streams (or PES for "Packetized Elementary Stream"). These PES notably contain, in the headers of the packets which form them, information making it possible to synchronize the video and audio streams which make up an audiovisual program. The packets forming a PES vary in size. Certain TS packets contain data making it possible to associate elementary streams so as to generate an audiovisual program, such as for example associating a video track with the appropriate audio track (s). For the sake of simplification, regularly called video stream an elementary packaged video stream, elsewhere in this document. The invention also applies in the same way to an audio stream corresponding to a packaged audio stream.

  The invention consists in substituting certain TS packets of the nominal stream with packets of the same structure and allowing the stream obtained, said first stream, to remain in conformity with the MPEG-2 standard, but the contents of which have been modified so as to alter the display.

  This substitution is carried out in two stages: - identification of the TS video packets to be modified; - analysis and modification of the content of TS packets to be modified.

  The identification of the contents of the TS packets consists in selecting, within the MPEG-2 TS stream, the packets carrying video information. This operation is similar to an operation for demultiplexing an MPEG-2 TS stream, well known to those skilled in the art.

  The second step is to analyze and modify the MPEG-2 video stream within TS packets of the nominal stream in MPEG-2 TS format.

  An MPEG-2 digital video stream consists of sequences of images (or planes or frames), grouped into groups of "Group Of Pictures" (GOPs). An image can be of type I (Intra), P 10 (Predicted), B (Bidirectional). The images I are the reference images, they are fully coded and are therefore of large size and do not contain information on the movement. The planes P are planes predicted from previous planes, whether I and / or P by motion vectors in a single direction, called forward. The planes B are said to be bidirectional, they are linked to the planes I and / or P preceding or following them by vectors of movement in the two temporal directions (in front and in back or 20 <backward "in English). Motion vectors represent two-dimensional vectors used for motion compensation, which provide the difference in coordinates between a part of the current image and a part of the reference image. An image is organized by 25 slices or slices, comprising macroblocks, themselves made up of blocks, containing elements describing the content of the video stream, for example the DC coefficients resulting from a frequency transformation and relating to the fundamental, this is that is to say the average value of the 30 coefficients of a block, or then the AC coefficients relating to the higher frequencies. The modification made to the video stream consists in using the correlations between the DC coefficients in an image I and the correlations between the images I, B and P, in order to be able to manipulate the appearance and the visual validity of the sequence to which the image I in question.

  Indeed, given that the I images are the 5 main carriers of information in an MPEG video sequence, any significant modification affecting these I images will necessarily have an impact on the validity of the sequence.

  Thus, the possibility of obtaining significant degradations of the images I without carrying out complicated manipulations is offered by the correlation between the coefficients DC in this image I. These coefficients, being strongly dependent on each other, because coded in differential, determine in much of the value of the following coefficients of the same kind. Thus, by modifying the information contained therein, we deeply modify the I images and consequently the entire MPEG sequence which follows this I image. Each macroblock of an I image contains six blocks, each of them starting with a DC coefficient.

  Four blocks correspond to the luminance (Y), and two to the chrominance (Cb and Cr) of the macroblock. The value written in a file of the MPEG stream representing a DC coefficient is in fact the difference between the true value of this coefficient and that of the corresponding coefficient which precedes and which, for the Y blocks, can be found in the same macroblock or in the previous macroblock, while for the blocks Cb and Cr it is always in the previous macroblock. A modification of one of these differences will automatically cause a modification of information in all the macroblocks which follow.

  This method is optimal for visual degradation of the MPEG video stream using the DC coefficients of the blocks, but it can also be applied to the AC coefficients. It is also possible to modify some of these AC coefficients randomly.

  In addition, the B and P images of the MPEG stream also contain coefficients AC and DC and are therefore also treated by the invention.

  The modification of the coefficients AC and DC 10 described above is carried out on the video content of the TS packets to be substituted. However, substituting one of these coefficients with a similar but random coefficient does not guarantee that the resulting MPEG-2 TS stream still meets the standard: the size of the inserted AC and 15 DC coefficients must be the same as the size of the coefficients initial. Otherwise, the multiplexing of the different streams making up the nominal MPEG-2 TS stream may no longer be correct. In addition, changing the size of the coefficients could change the bit rate of the 20 MPEG-2 TS stream, which can cause major problems when broadcast over conventional networks such as satellite or cable. This is why the inserted random coefficients must have exactly the same binary size as the original coefficients. In the following, we will explain how to effect modifications of DC coefficients and of AC coefficients, without changing the size of the binary words on which they are coded. It is however perfectly possible to modify another binary field of the video stream in a complementary manner, so as to further degrade the display of the modified video program, these modifications keeping the stream in accordance with the standard.

  The DC coefficients are coded on two binary fields in MPEG-2: a "dct_dcsize" field which we will call "size", and a "dctdcdifferential" field which we will call <differential ". The first field 5" size "allows to indicate the number of bits on which the "differential" value is coded. Substituting a DC coefficient by a random coefficient is therefore possible by leaving the "size" field unchanged, and replacing the "differential" field with a random binary word of length " cut ".

  The AC coefficients are coded in the form of a list of couples (run, level). A "run" (series in French) being the number of null AC coefficients preceding a coefficient AC of value "level" (level in French). 15 Thus, the series of coefficients AC 0, 0, 1, 0, 0, 0, 5 is written in the form (2,1), (3,5). These pairs (run, level) are also written in the bit stream using a variable length coding technique: the largest occurrence couples are coded on a lower 20-bit number than the less frequently encountered couples.

  Couples for which no variable length code has been provided (because their frequency of appearance is too low) are written to the stream using an <escape code "or escape code. technique consists of writing a predefined binary word in the stream, indicating to the decoder that the "run" and the "level" it is reading are coded separately and independently, on binary words of fixed length (6 bits for "run" and 12 for signed "level") 30 just after the escape code. There are generally several possible pairs ("run", "level") for a given bit length. binary 0100 and 0101 correspond to two different couples (<run "," level "). Replacing the AC coefficients of an MPEG-2 video stream with random values therefore consists in choosing an authorized binary word of the same size as the binary word to be modified, and writing it in its place. The MPEG-2 standard defines 5 two possible tables of correspondence between "run", "level" pairs and binary words (table B-14 and B-15 of the MPEG-2 video standard); to find a pair of the same size as the pair to be substituted, it suffices to browse the adapted table and find a suitable pair 10. This is not always possible for the shortest binary words, in this case the couple should be left unchanged. In the case where the pair (<"run", "level">) is written using an <escape code "or escape code, we replace the binary words corresponding to" run "and" level "by random values, on the same number of bits.

  The modifications made to the elementary video stream are only made within certain TS packets, chosen according to their characteristics and their content. Several exemplary embodiments are described below.

  In a first embodiment, the MPEG-2 TS stream is analyzed so as to be able to identify and process the binary words corresponding to the coefficients DC 25 and AC, contained in the TS packets. One or more video streams are processed. The remainder of the document describes the process used to process a video stream. In this exemplary embodiment, all of the DC and AC coefficients identified are modified in the manner described above. The method keeps in memory an original copy of the TS packet that it is processing. When the system has finished processing a given TS packet, it decides either to take the original TS packet (unmodified) and write it to the output stream (otherwise called first stream), or to write the packet that he just got by modifying the original package. Said first stream obtained therefore has TS packets whose contents have been entirely modified, and TS packets whose contents are unchanged.

  If several video streams are processed simultaneously, the processing to be carried out is the same for each video stream, and the various processing operations are carried out independently.

  The choice of modified TS packets to be written at the output of the analysis and modification module depends on the quantity of modifications to be made to the flow. It also depends on the video content processed. Thus, a modification made to a long binary word is more degrading visually than a modification made to a short binary word because in the case of a long binary word, the number of binary words of the same size available is greater. Thus, there is a greater chance that the inserted value is very far from the original value for a long word than for a short word. When deciding whether or not to write the TS packet as output, the system tests the average length of the coefficients which have been modified within the packet, and if this is greater than a threshold, it writes the modified TS packet in output, otherwise, it is the original unmodified TS packet which it writes at the output of the analysis and modification system.

  In a variant of this exemplary embodiment, only the TS packets containing information relating to the images I of the video stream are processed and written in said first stream.

  In another variant of this exemplary embodiment, the TS packets containing information relating to the I and P or B images of the video stream are processed and written in said first stream.

  In another variant of this exemplary embodiment, only the TS packets containing information relating to the beginnings of the "slices" or slices composing the images I of the video stream are processed and written in said first stream.

  In another exemplary embodiment, the coefficients AC and DC are not all modified within the video stream processed. The analysis and modification system uses a modification or substitution control "switch". When the system starts processing a given TS packet, it decides whether or not to modify it randomly. If he decides to modify it, he places his switch in the "modification" position; in this case, all of the following AC and DC coefficients are modified until the switch returns to the "copy" position. The system goes to the "copy" position when it starts processing a TS packet which it does not wish to modify.

  In all the exemplary embodiments, the original TS packets, corresponding to packets modified in the first modified stream, are kept in a second stream of any format in the portal (12) and 25 are not transmitted to the module (8 ).

  When reading the binary sequence, a conventional MPEG decoder identifies the beginning of an encoded image, then the type of the image. To avoid any confusion between a standard decoder box often called "Set Top Box or 30 STB", the standard MPEG decoder will be called "Player" (g Player "or <Viewer") in the rest of the document. This Reader can be produced physically (or "hardware") and / or computerized ("software").

  The MPEG player successively decodes each macroblock of the image. The image is reconstructed when all of its macroblocks have been processed. If it is an image I, it constitutes a reference image for the subsequent images and it is stored in place of the oldest reference image. The images are thus available, in digital form, for further processing and display, depending on the application.

  Certain TS packets of the audiovisual stream are therefore kept in the portal (12). On the other hand, in place of the non-transmitted TS packets, the device according to the invention will insert erroneous TS packets of the same nature and size as the packets removed and stored in the portal (12) so that the standard MPEG player of the 15 module (8) is not disturbed by these modifications that it will ignore and reconstruct as output an MPEG output stream which will be not correct from the visual point of view for a human being but correct from the point of view of MPEG format.

  The MPEG player of the set-top box (8) is a standard MPEG player and is in no way modified or affected by the changes made within the MPEG stream.

  As shown in FIG. 1, the connection interface (9) is connected to an extended telecommunications network (10), directly or by a local network serving as an access network and consists for example of a communication interface. subscriber line (Analog or digital telephone network, DSL, BLR, GSM, GPRS, UMTS, etc.).

  Thus, the audiovisual programs are conventionally broadcast in multi-broadcast mode 30 ("broadcast") via the broadband transmission network (4) of the hertzian, cable, satellite, digital hertzian, DSL, etc. type. from the server (1) directly via the link (3bis) or via the portal (12) via the link (2) and (3) to the decoder module (8) through the link (5).

  Each audiovisual program thus broadcast can be encrypted or not, and, in accordance with the present invention, the MPEG type streams comprise TS packets substituted by packets whose contents have been modified as described above. Depending on the parameters chosen by the user or on the information transmitted by the broadcasting server, certain audiovisual programs thus modified and incomplete are recorded in the hard disk of box 10 (8).

  When the user wishes to view an audiovisual program thus recorded in the hard disk of his box (8), he requests it in a conventional manner by means of a remote control connected to his box (8) which then automatically connects to the portal. (12) via the link (9) of the local network or direct access type and through the telecommunications network (10) itself connected to the portal (12) via the link (11). Throughout the visualization of the audiovisual program, the links (9) and 20 (11) remain established and allow the box (8) to receive the functions and parameters for reordering and restoring the modified TS packets, as well as the original TS packets kept in the portal (12). TS packets thus transmitted are never recorded in the hard disk of the box (8) because the video stream reconstituted by the synthesis module is directly displayed on the display screen (6) via the link (7) after being processed by the Reader of the box (8) from its volatile local memory. Once processed and displayed, the original TS packets which have just been transmitted by the portal (12) are erased from the local volatile memory of the box (8).

  Whenever the user wants to watch a program recorded in the hard disk of the box (8) the box (8) will automatically connect to the portal (12). The reconstruction of the original MPEG stream in the video interface arrangement (8) is lossless.

  Similarly, when the user pauses, the transmission of said second stream, containing the original TS packets, coming from the portal (12) will be interrupted until resumption of viewing, thus guaranteeing that all the information of 'an audiovisual program is not found in the box (8) at a given time and thus avoiding a malicious person to make pirated copies of these recordings.

  In a particular embodiment, said second stream containing the information enabling the audiovisual program to be reconstituted and displayed by the user is structured so that it complies with the MPEG-2 TS standard.

  Although the present invention is more particularly focused on the video streams included in the MPEG-2 TS streams, it is understood that any interactive multimedia information and any interactive data multiplexed in an MPEG-2 TS stream can be processed by the present arrangement and the present system, the MPEG type video data being the most elaborate. The present invention will be better understood from the following description presenting the physical basis of the present invention and with reference to FIG. 2 of the appended drawing 30 representing a preferred embodiment of the latter as a non-limiting example of embodiment particularly well suited for cable and satellite networks.

  The complete MPEG-2 TS stream (101) is analyzed by the analysis device (121) of the portal (12) and will thus be separated into an MPEG-2 TS type stream, said first stream, but some TS packets of which will have have been substituted and which will be sent via the outlet (122) of the portal to the broadband transmission broadcasting network (4).

  The other part of the modified MPEG stream, said second stream, will be stored in the buffer memory (123) of the portal (12). For each MPEG stream thus broadcast, the portal (12) will keep in a buffer memory (123) the modifications which have been made to this MPEG-2 TS stream by the analyzer (121) of the portal (12) as well as the packets Original TS. It is specified that, for the same MPEG-2 TS input stream (101), the processing of the stream may be different for each user (8) and / or for each group of users (8). Thus, the buffer memory (123) of the portal (12) includes a different memory area for each user.

  In the example which follows, only the 20 TS video packets containing the start of a slice (or slice) of an image I, the content of which relates to an image I, are modified.

  Now let's describe in detail the different steps for a user.

  The portal (121) has chosen the MPEG-2 TS stream (101) which it will have to send to the user (8) to be watched on its television screen (6). This user is connected to a digital cable broadcasting network (4) and to an ADSL telecommunications network (10).

  The analysis system (121) of the portal (12) will therefore read the incoming MPEG-2 TS stream (101) and, each time it detects a TS packet to be modified, it analyzes its content. This analysis allows it to recognize in the code the coefficients DC and AC, and to substitute them by random values without changing their size, within the TS packet, in order to make the images (and consequently the sequence) unacceptable from the point 5 human visual perception. The real TS packets will be stored in the output buffer (123), which will later allow the reconstitution of the starting sequence in the box (8), following the reverse diagram.

  The analysis and modification system (121) 10 then writes the real TS packet into the buffer (123). The analysis system (121) continues its analysis until the end of the MPEG input stream.

  The new modified MPEG stream is then recorded in the output buffer (122) to be broadcast on the broadcasting network (4) through the link (3).

  The substituted TS packets of the incoming MPEG-2 TS stream (101) are stored in the buffer (123) of the portal (12).

  Meanwhile, and in a completely unsynchronized manner, the modified MPEG-2 TS output stream coming from the output buffer (122) of the portal (12) is broadcast via the broadband network (4) to one or more users. (8) and stored on the hard disk (85).

  The phase described above corresponds to the first phase of preparation of the MPEG-2 TS stream by the portal (12), its transmission via the broadband network (4) and its recording in a decoder (8). This decoder can then display this MPEG-2 TS stream recorded on its hard disk (85). For this, the synthesis system (87) of the decoder (8) will read the MPEG-2 TS file from its hard disk (85) and will send it to a conventional MPEG-2 TS player (81). If no additional data is received by the synthesis system (87), then the MPEG-2 TS stream which reaches the player (81) is processed and displayed as it is, which causes a significant distortion of the display on the screen. 'display screen (6). Indeed, the modified TS packets which are processed by the synthesis system (87) do not correspond to the TS packets which are necessary for correct visualization, since certain DC and AC coefficients have been replaced by random coefficients. On the other hand, since the recorded stream is indeed an MPEG-2 TS type stream, the player (81) makes no difference and displays the information 10 on the output screen (6) which appears well as data from a MPEG-2 TS video streams but totally inconsistent for the human eye looking at the screen (6). Any copy of the MPEG-2 TS stream coming from the hard disk (85) of the box (8) will produce the same visual effect when it is restored by any MPEG player. When the user of the decoder (8) really wants to view on his screen (6) the audiovisual program recorded on his hard disk (85), he requests it from the synthesis system (87) with his remote control 20 as he would with a VCR or DVD player presenting a menu on its television screen. The synthesis system (87) then makes a request to the hard disk (85) and begins to analyze the modified MPEG-2 TS stream coming from the hard disk (85) via the read buffer 25 (83). The synthesis system (87) then establishes a link with the portal (12) via the telecommunications network (10) which is in our example a DSL link. Once this connection has been established, and for the entire duration of viewing the film or the audiovisual program, the synthesis system (87) sends the original TS packets and digital information from the server's buffer memory (123). corresponding to the stream recorded on the hard disk (85). This data reaches the synthesis system (87) via the input buffer memory (86) and is temporarily stored in the synthesis system (87). From the modified MPEG-2 TS stream which arrives via the buffer (83) and from the original TS packets and the associated data which arrives via the buffer (86), the synthesis system (87) reconstitutes inversely to the previously described analysis process, the original MPEG-2 TS stream and sends the new thus reconstructed MPEG-2 TS stream to the player (81) to be displayed correctly on the screen (6).

  Advantageously, as soon as they are used, the original TS packets coming from the portal (12) via said second stream, and the associated data are erased from the synthesis system (87).

  Advantageously, before the portal (12) sends the original TS packets and associated data from its buffer (123), the portal (12) has verified that the user of the housing (8) is indeed authorized to receive said second flux.

Claims (6)

  1 - Method for the distribution of audiovisual sequences according to a nominal stream format respecting the MPEG-2 TS standard, composed of a succession of TS packets, characterized in that one proceeds, before transmission to the client equipment , to an analysis of the flow to generate a first flow conforming to the MPEG-2 TS format, modified in that all or part of the TS packets constituting it have been replaced by packets of the same structure but whose content has been modified, and a second stream of any format 10, comprising the digital information able to allow the reconstruction of said nominal stream, then to transmit separately the two streams thus generated from the server to the recipient equipment, and in that one calculates on l recipient equipment a summary 15 of a stream in nominal format as a function of said first stream and said second stream.   2 - Method for the distribution of audiovisual sequences according to claim 1, characterized in that said substituted TS packets contain information relating to one or more video streams included in the TS stream.   3 - A method for the distribution of audiovisual sequences according to claim 1 or 2, characterized in that said analysis of the flow includes a step of determining the TS packets to be substituted as a function of their video content.   4 - Method for the distribution of audiovisual sequences according to one of the preceding claims, characterized in that said second stream contains the substituted TS packets originating from the nominal stream.   - Method for the distribution of audiovisual sequences according to one of the preceding claims, characterized in that said second stream respects the MPEG-2 TS format.   6. Method for the distribution of audiovisual sequences according to one of the preceding claims, characterized in that said first stream has the same size in bytes as the original stream in MPEG-2 TS format.   7 - Equipment for the manufacture of a video stream for the implementation of the method according to one of claims 1 to 6, comprising at least one multimedia server containing the original video sequences and characterized in that it comprises a device analyzing the video stream from said server to generate said two streams.   8 - Equipment for the exploitation of a video stream for the implementation of the method according to one of claims 1 to 6, comprising a standard player of 25 MPEG-2 TS streams, at least one recording interface [disc hard] intended for storing the content of said first stream and at least one display interface, characterized in that it includes means for recomposing the original stream from said two streams. 9 9 A system for the transmission of a video stream comprising equipment for producing a video stream according to claim 7, at least one equipment for operating a video stream according to claim 8 and at least one communication network between production equipment and operating equipment (s).